The present invention relates to the field of automatic transmissions and, more particularly, to a high performance intermediate servo assembly for use in General Motors 200 4R transmissions (hereinafter “GM transmissions”).
Automatic transmission systems of the prior art have a hydraulic circuit sub-system which includes at least a hydraulic pump, a valve body having fluid conducting passages or circuits, input and exhaust ports formed within the fluid circuits, and a plurality of valves comprised of generally cylindrical pistons having control lands formed thereon, which alternately open and close the ports to the fluid circuits to regulate the flow and pressure of automatic transmission fluid (hereinafter “ATF”) to hydraulically actuate various components of the transmission. It will be understood that in describing hydraulic circuits, ATF usually changes names when it passes through an orifice or control valve in a specific fluid circuit.
In the GM transmissions the intermediate servo assembly is applied in 2nd gear and is used as an accumulator in 3rd gear. An accumulator is a spring-loaded device that absorbs a certain amount of apply fluid pressure to cushion the application of a clutch band against fluid shock to control shift feel and to prevent damage to these components.
In high performance applications it is desirable to increase the hydraulic fluid capacity and fluid pressure generated by the intermediate servo assembly to increase the holding capacity of the clutch band under high load conditions.
There are known prior art patents in the field and their discussion follows. U.S. Pat. No. 4,601,233 to Sugano discloses a hydraulic servo device with a built-in accumulator comprising a servo piston fit in a servo cylinder and connected via a stem to a band brake, an accumulator piston, and an accumulator piston spring biases the accumulator piston toward the servo piston. A servo release pressure acts on a pressure acting area, a servo apply pressure acts on a pressure acting area, and an accumulator pressure acts on a pressure acting area in the operation thereof.
Another example of this type of servo is shown in U.S. Pat. No. 4,787,494 to Ogasawara et al., which discloses a hydraulic operating unit for a friction-engaging band of a transmission that permits the introduction of a low-pressure working fluid to allow forcible disengagement of the hydraulic operation unit to work against the fluid, which would normally urge the unit into engagement.
U.S. Pat. No. 4,388,986 to Umezawa discloses a speed change control for an automatic transmission having a planetary gear mechanism and a brake band for altering the transmission ratio. The system comprises a servo-piston, a piston rod secured to the servo-piston and to the brake band, and an accumulator piston axially slidably disposed in the bore of the servo-piston.
U.S. Pat. No. 5,944,627 to Darling-Owen and also owned by the applicant discloses a replacement servo mechanism for applying a band in an automatic transmission wherein the servo assembly includes two servo piston members, a major piston member and a minor piston member, to which an apply pin is connected.
It is also known in the prior art to adapt the original equipment manufacture (hereinafter “OEM”) intermediate servo for high performance applications by providing an aftermarket intermediate servo kit. A commercially available kit of the type marketed by Shift Technology Products, a division of Superior Transmission Parts, Inc., Tallahassee, Fla., has been designed for this purpose (FIG. 3) and is described hereinafter in further detail. For the high performance application, the user of this prior art kit is instructed by the manufacturer to disable the function of the integrated accumulator. All of the remaining OEM components are effectively used as spacers in reassembly. However, reassembly of the intermediate servo in such a manner creates mismatched part surfaces causing inaccurate operation and the potential for noise and abnormal wear.
While these devices fulfill their respective, particular objectives and requirements, the aforementioned patents do not disclose the high performance intermediate servo assembly of the present invention, which substantially departs from the conventional concepts and designs of the prior art.